pathfinder_simd: Please detect unstable features rather than the nightly channel

[bjorn3]

Or just forgo unstable feature detection at all and have an explicit nightly opt-in. Just because you are on nightly doesn't mean using nightly features will compile. The feature could have been changed in an incompatible way, or could simply have been disabled by the user through -Zallow-features= or RUSTC_BOOTSTRAP=-1.

[jdm]

Explicit opt-in seems like the best choice here. Automatic detection feels like a footgun to me.